But there is an important point here, which is that the more teams using the 3-4, the less effective it is. The same cannot be said of the 4-3, which is definitely effective (or not) as a function of personnel and coaching.

There's a second important point: 3-4 defensive ends are cheaper, relatively speaking than 4-3 ends, since by and large all they are asked to do is mind their gaps and stop the run. If they can also sack the QB, great, but that's not why they were hired. And since you have one less defensive linemen, the defense as a whole is cheaper as a result. The Steelers are likely to stick with a formula that not only produces results, but saves money.